311P/PANSTARRS also known as P/2013 P5 (PANSTARRS) is an asteroid (or main-belt comet) discovered by the Pan-STARRS telescope on 27 August 2013.[4] Observations made by the Hubble Space Telescope revealed that it had six comet-like tails.[5] The tails are suspected to be streams of material ejected by the asteroid as a result of a rubble pile asteroid spinning fast enough to remove material from it.[2] This is similar to 331P/Gibbs, which was found to be a quickly-spinning rubble pile as well.

Three-dimensional models constructed by Jessica Agarwal of the Max Planck Institute for Solar System Research in Lindau, Germany, showed that the tails could have formed by a series of periodic impulsive dust-ejection events,[6] radiation pressure from the sun then stretched the dust into streams.[5]

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The asteroid has a radius of about 240 meters (790 ft).[2] The first images taken by Pan-STARRS revealed that the object had an unusual appearance: asteroids generally appear as small points of light, but P/2013 P5 was identified as a fuzzy-looking object by astronomers.[7] The multiple tails were observed by the Hubble Space Telescope on 10 September 2013, Hubble later returned to the asteroid on 23 September, its appearance had totally changed. It looked as if the entire structure had swung around.[8] The Hubble Space Telescope continued to track the object through 11 February 2014.[9] The comet-like appearance has resulted in the asteroid being named as a comet. The object has a low orbital inclination and always stays outside the orbit of Mars.[1]

1.
Hubble Space Telescope
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The Hubble Space Telescope is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. Although not the first space telescope, Hubble is one of the largest and most versatile, with a 2. 4-meter mirror, Hubbles four main instruments observe in the near ultraviolet, visible, and near infrared spectra. Hubbles orbit outside the distortion of Earths atmosphere allows it to take extremely high-resolution images, Hubble has recorded some of the most detailed visible light images ever, allowing a deep view into space and time. Many Hubble observations have led to breakthroughs in astrophysics, such as determining the rate of expansion of the universe. The HST was built by the United States space agency NASA, the Space Telescope Science Institute selects Hubbles targets and processes the resulting data, while the Goddard Space Flight Center controls the spacecraft. Space telescopes were proposed as early as 1923, Hubble was funded in the 1970s, with a proposed launch in 1983, but the project was beset by technical delays, budget problems, and the Challenger disaster. When finally launched in 1990, Hubbles main mirror was found to have been ground incorrectly, the optics were corrected to their intended quality by a servicing mission in 1993. Hubble is the telescope designed to be serviced in space by astronauts. After launch by Space Shuttle Discovery in 1990, five subsequent Space Shuttle missions repaired, upgraded, the fifth mission was canceled on safety grounds following the Columbia disaster. However, after spirited public discussion, NASA administrator Mike Griffin approved the fifth servicing mission, the telescope is operating as of 2017, and could last until 2030–2040. Its scientific successor, the James Webb Space Telescope, is scheduled for launch in 2018, the history of the Hubble Space Telescope can be traced back as far as 1946, to the astronomer Lyman Spitzers paper Astronomical advantages of an extraterrestrial observatory. In it, he discussed the two advantages that a space-based observatory would have over ground-based telescopes. First, the resolution would be limited only by diffraction, rather than by the turbulence in the atmosphere. Second, a telescope could observe infrared and ultraviolet light. Spitzer devoted much of his career to pushing for the development of a space telescope, space-based astronomy had begun on a very small scale following World War II, as scientists made use of developments that had taken place in rocket technology. An orbiting solar telescope was launched in 1962 by the United Kingdom as part of the Ariel space program, oAO-1s battery failed after three days, terminating the mission. It was followed by OAO-2, which carried out observations of stars and galaxies from its launch in 1968 until 1972. The continuing success of the OAO program encouraged increasingly strong consensus within the community that the LST should be a major goal

2.
Minor planet
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A minor planet is an astronomical object in direct orbit around the Sun that is neither a planet nor exclusively classified as a comet. Minor planets can be dwarf planets, asteroids, trojans, centaurs, Kuiper belt objects, as of 2016, the orbits of 709,706 minor planets were archived at the Minor Planet Center,469,275 of which had received permanent numbers. The first minor planet to be discovered was Ceres in 1801, the term minor planet has been used since the 19th century to describe these objects. The term planetoid has also used, especially for larger objects such as those the International Astronomical Union has called dwarf planets since 2006. Historically, the asteroid, minor planet, and planetoid have been more or less synonymous. This terminology has become complicated by the discovery of numerous minor planets beyond the orbit of Jupiter. A Minor planet seen releasing gas may be classified as a comet. Before 2006, the IAU had officially used the term minor planet, during its 2006 meeting, the IAU reclassified minor planets and comets into dwarf planets and small Solar System bodies. Objects are called dwarf planets if their self-gravity is sufficient to achieve hydrostatic equilibrium, all other minor planets and comets are called small Solar System bodies. The IAU stated that the minor planet may still be used. However, for purposes of numbering and naming, the distinction between minor planet and comet is still used. Hundreds of thousands of planets have been discovered within the Solar System. The Minor Planet Center has documented over 167 million observations and 729,626 minor planets, of these,20,570 have official names. As of March 2017, the lowest-numbered unnamed minor planet is 1974 FV1, as of March 2017, the highest-numbered named minor planet is 458063 Gustavomuler. There are various broad minor-planet populations, Asteroids, traditionally, most have been bodies in the inner Solar System. Near-Earth asteroids, those whose orbits take them inside the orbit of Mars. Further subclassification of these, based on distance, is used, Apohele asteroids orbit inside of Earths perihelion distance. Aten asteroids, those that have semi-major axes of less than Earths, Apollo asteroids are those asteroids with a semimajor axis greater than Earths, while having a perihelion distance of 1.017 AU or less. Like Aten asteroids, Apollo asteroids are Earth-crossers, amor asteroids are those near-Earth asteroids that approach the orbit of Earth from beyond, but do not cross it

3.
Asteroid
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Asteroids are minor planets, especially those of the inner Solar System. The larger ones have also been called planetoids and these terms have historically been applied to any astronomical object orbiting the Sun that did not show the disc of a planet and was not observed to have the characteristics of an active comet. As minor planets in the outer Solar System were discovered and found to have volatile-based surfaces that resemble those of comets, in this article, the term asteroid refers to the minor planets of the inner Solar System including those co-orbital with Jupiter. There are millions of asteroids, many thought to be the remnants of planetesimals. The large majority of known asteroids orbit in the belt between the orbits of Mars and Jupiter, or are co-orbital with Jupiter. However, other orbital families exist with significant populations, including the near-Earth objects, individual asteroids are classified by their characteristic spectra, with the majority falling into three main groups, C-type, M-type, and S-type. These were named after and are identified with carbon-rich, metallic. The size of asteroids varies greatly, some reaching as much as 1000 km across, asteroids are differentiated from comets and meteoroids. In the case of comets, the difference is one of composition, while asteroids are composed of mineral and rock, comets are composed of dust. In addition, asteroids formed closer to the sun, preventing the development of the aforementioned cometary ice, the difference between asteroids and meteoroids is mainly one of size, meteoroids have a diameter of less than one meter, whereas asteroids have a diameter of greater than one meter. Finally, meteoroids can be composed of either cometary or asteroidal materials, only one asteroid,4 Vesta, which has a relatively reflective surface, is normally visible to the naked eye, and this only in very dark skies when it is favorably positioned. Rarely, small asteroids passing close to Earth may be visible to the eye for a short time. As of March 2016, the Minor Planet Center had data on more than 1.3 million objects in the inner and outer Solar System, the United Nations declared June 30 as International Asteroid Day to educate the public about asteroids. The date of International Asteroid Day commemorates the anniversary of the Tunguska asteroid impact over Siberia, the first asteroid to be discovered, Ceres, was found in 1801 by Giuseppe Piazzi, and was originally considered to be a new planet. In the early half of the nineteenth century, the terms asteroid. Asteroid discovery methods have improved over the past two centuries. This task required that hand-drawn sky charts be prepared for all stars in the band down to an agreed-upon limit of faintness. On subsequent nights, the sky would be charted again and any moving object would, hopefully, the expected motion of the missing planet was about 30 seconds of arc per hour, readily discernible by observers

4.
Main-belt comet
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Main-belt comets are bodies orbiting within the asteroid belt that have shown comet-like activity during part of their orbit. The Jet Propulsion Laboratory defines a main-belt asteroid as an asteroid with an axis of more than 2 AU but less than 3.2 AU. The first main-belt comet discovered is 7968 Elst–Pizarro and it was discovered in 1979 and was found to have a tail by Eric Elst and Guido Pizarro in 1996 and given the cometary designation 133P/Elst-Pizarro. Although quite a few comets have semimajor axes well within Jupiters orbit, main-belt comets differ in having small eccentricities. The first three identified main-belt comets all orbit within the part of the asteroid belt. It is not known how an outer Solar System body like the other comets could have made its way into a low-eccentricity orbit typical of the asteroid belt, some main-belt comets display a cometary dust tail only for a part of their orbit near perihelion. Activity in 133P/Elst–Pizarro is recurrent, having been observed at each of the last three perihelia, the activity persists for a month or several out of each 5-6 year orbit, and is presumably due to ice being uncovered by minor impacts in the last 100 to 1000 years. These impacts are suspected to excavate these subsurface pockets of volatile material helping to expose them to solar radiation, observations of Scheila indicated that large amounts of dust were kicked up by the impact of another asteroid of approximately 35 meters in diameter. In October 2013, observations of P/2013 R3, taken with the 10.4 m Gran Telescopio Canarias on the island of La Palma showed that this comet was breaking apart. The brightest A fragment was also detected at the position in CCD images obtained at the 1.52 m telescope of the Sierra Nevada Observatory in Granada on October 12. NASA reported on a series of images taken by the Hubble Space Telescope between October 29,2013 and January 14,2014 that show the separation of the four main bodies. The Yarkovsky–OKeefe–Radzievskii–Paddack effect, caused by sunlight, increased the rate until the centrifugal force caused the rubble pile to separate. The term main-belt comet is a based on orbit and the presence of an extended morphology. It does not imply that these objects are comets or that the material surrounding their nuclei was ejected by the sublimation of volatiles, identified members of this morphology class include, Centaur Extinct comet Henry Hsiehs Main-Belt Comets page has extensive details on Main-belt comets David Jewitt. J. Licandro New images obtained with the GTC

5.
Perihelion and aphelion
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The perihelion is the point in the orbit of a celestial body where it is nearest to its orbital focus, generally a star. It is the opposite of aphelion, which is the point in the orbit where the body is farthest from its focus. The word perihelion stems from the Ancient Greek words peri, meaning around or surrounding, aphelion derives from the preposition apo, meaning away, off, apart. According to Keplers first law of motion, all planets, comets. Hence, a body has a closest and a farthest point from its parent object, that is, a perihelion. Each extreme is known as an apsis, orbital eccentricity measures the flatness of the orbit. Because of the distance at aphelion, only 93. 55% of the solar radiation from the Sun falls on a given area of land as does at perihelion. However, this fluctuation does not account for the seasons, as it is summer in the northern hemisphere when it is winter in the southern hemisphere and vice versa. Instead, seasons result from the tilt of Earths axis, which is 23.4 degrees away from perpendicular to the plane of Earths orbit around the sun. Winter falls on the hemisphere where sunlight strikes least directly, and summer falls where sunlight strikes most directly, in the northern hemisphere, summer occurs at the same time as aphelion. Despite this, there are larger land masses in the northern hemisphere, consequently, summers are 2.3 °C warmer in the northern hemisphere than in the southern hemisphere under similar conditions. Apsis Ellipse Solstice Dates and times of Earths perihelion and aphelion, 2000–2025 from the United States Naval Observatory

6.
Astronomical unit
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The astronomical unit is a unit of length, roughly the distance from Earth to the Sun. However, that varies as Earth orbits the Sun, from a maximum to a minimum. Originally conceived as the average of Earths aphelion and perihelion, it is now defined as exactly 149597870700 metres, the astronomical unit is used primarily as a convenient yardstick for measuring distances within the Solar System or around other stars. However, it is also a component in the definition of another unit of astronomical length. A variety of symbols and abbreviations have been in use for the astronomical unit. In a 1976 resolution, the International Astronomical Union used the symbol A for the astronomical unit, in 2006, the International Bureau of Weights and Measures recommended ua as the symbol for the unit. In 2012, the IAU, noting that various symbols are presently in use for the astronomical unit, in the 2014 revision of the SI Brochure, the BIPM used the unit symbol au. In ISO 80000-3, the symbol of the unit is ua. Earths orbit around the Sun is an ellipse, the semi-major axis of this ellipse is defined to be half of the straight line segment that joins the aphelion and perihelion. The centre of the sun lies on this line segment. In addition, it mapped out exactly the largest straight-line distance that Earth traverses over the course of a year, knowing Earths shift and a stars shift enabled the stars distance to be calculated. But all measurements are subject to some degree of error or uncertainty, improvements in precision have always been a key to improving astronomical understanding. Improving measurements were continually checked and cross-checked by means of our understanding of the laws of celestial mechanics, the expected positions and distances of objects at an established time are calculated from these laws, and assembled into a collection of data called an ephemeris. NASAs Jet Propulsion Laboratory provides one of several ephemeris computation services, in 1976, in order to establish a yet more precise measure for the astronomical unit, the IAU formally adopted a new definition. Equivalently, by definition, one AU is the radius of an unperturbed circular Newtonian orbit about the sun of a particle having infinitesimal mass. As with all measurements, these rely on measuring the time taken for photons to be reflected from an object. However, for precision the calculations require adjustment for such as the motions of the probe. In addition, the measurement of the time itself must be translated to a scale that accounts for relativistic time dilation

7.
Orbital eccentricity
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The orbital eccentricity of an astronomical object is a parameter that determines the amount by which its orbit around another body deviates from a perfect circle. A value of 0 is an orbit, values between 0 and 1 form an elliptical orbit,1 is a parabolic escape orbit. The term derives its name from the parameters of conic sections and it is normally used for the isolated two-body problem, but extensions exist for objects following a rosette orbit through the galaxy. In a two-body problem with inverse-square-law force, every orbit is a Kepler orbit, the eccentricity of this Kepler orbit is a non-negative number that defines its shape. The limit case between an ellipse and a hyperbola, when e equals 1, is parabola, radial trajectories are classified as elliptic, parabolic, or hyperbolic based on the energy of the orbit, not the eccentricity. Radial orbits have zero angular momentum and hence eccentricity equal to one, keeping the energy constant and reducing the angular momentum, elliptic, parabolic, and hyperbolic orbits each tend to the corresponding type of radial trajectory while e tends to 1. For a repulsive force only the trajectory, including the radial version, is applicable. For elliptical orbits, a simple proof shows that arcsin yields the projection angle of a circle to an ellipse of eccentricity e. For example, to view the eccentricity of the planet Mercury, next, tilt any circular object by that angle and the apparent ellipse projected to your eye will be of that same eccentricity. From Medieval Latin eccentricus, derived from Greek ἔκκεντρος ekkentros out of the center, from ἐκ- ek-, eccentric first appeared in English in 1551, with the definition a circle in which the earth, sun. Five years later, in 1556, a form of the word was added. The eccentricity of an orbit can be calculated from the state vectors as the magnitude of the eccentricity vector, e = | e | where. For elliptical orbits it can also be calculated from the periapsis and apoapsis since rp = a and ra = a, where a is the semimajor axis. E = r a − r p r a + r p =1 −2 r a r p +1 where, rp is the radius at periapsis. For Earths annual orbit path, ra/rp ratio = longest_radius / shortest_radius ≈1.034 relative to center point of path, the eccentricity of the Earths orbit is currently about 0.0167, the Earths orbit is nearly circular. Venus and Neptune have even lower eccentricity, over hundreds of thousands of years, the eccentricity of the Earths orbit varies from nearly 0.0034 to almost 0.058 as a result of gravitational attractions among the planets. The table lists the values for all planets and dwarf planets, Mercury has the greatest orbital eccentricity of any planet in the Solar System. Such eccentricity is sufficient for Mercury to receive twice as much solar irradiation at perihelion compared to aphelion, before its demotion from planet status in 2006, Pluto was considered to be the planet with the most eccentric orbit

8.
Mean anomaly
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In celestial mechanics, the mean anomaly is an angle used in calculating the position of a body in an elliptical orbit in the classical two-body problem. Define T as the time required for a body to complete one orbit. In time T, the radius vector sweeps out 2π radians or 360°. The average rate of sweep, n, is then n =2 π T or n =360 ∘ T, define τ as the time at which the body is at the pericenter. From the above definitions, a new quantity, M, the mean anomaly can be defined M = n, because the rate of increase, n, is a constant average, the mean anomaly increases uniformly from 0 to 2π radians or 0° to 360° during each orbit. It is equal to 0 when the body is at the pericenter, π radians at the apocenter, if the mean anomaly is known at any given instant, it can be calculated at any later instant by simply adding n δt where δt represents the time difference. Mean anomaly does not measure an angle between any physical objects and it is simply a convenient uniform measure of how far around its orbit a body has progressed since pericenter. The mean anomaly is one of three parameters that define a position along an orbit, the other two being the eccentric anomaly and the true anomaly. Define l as the longitude, the angular distance of the body from the same reference direction. Thus mean anomaly is also M = l − ϖ, mean angular motion can also be expressed, n = μ a 3, where μ is a gravitational parameter which varies with the masses of the objects, and a is the semi-major axis of the orbit. Mean anomaly can then be expanded, M = μ a 3, and here mean anomaly represents uniform angular motion on a circle of radius a

9.
Orbital inclination
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Orbital inclination measures the tilt of an objects orbit around a celestial body. It is expressed as the angle between a plane and the orbital plane or axis of direction of the orbiting object. For a satellite orbiting the Earth directly above the equator, the plane of the orbit is the same as the Earths equatorial plane. The general case is that the orbit is tilted, it spends half an orbit over the northern hemisphere. If the orbit swung between 20° north latitude and 20° south latitude, then its orbital inclination would be 20°, the inclination is one of the six orbital elements describing the shape and orientation of a celestial orbit. It is the angle between the plane and the plane of reference, normally stated in degrees. For a satellite orbiting a planet, the plane of reference is usually the plane containing the planets equator, for planets in the Solar System, the plane of reference is usually the ecliptic, the plane in which the Earth orbits the Sun. This reference plane is most practical for Earth-based observers, therefore, Earths inclination is, by definition, zero. Inclination could instead be measured with respect to another plane, such as the Suns equator or the invariable plane, the inclination of orbits of natural or artificial satellites is measured relative to the equatorial plane of the body they orbit, if they orbit sufficiently closely. The equatorial plane is the perpendicular to the axis of rotation of the central body. An inclination of 30° could also be described using an angle of 150°, the convention is that the normal orbit is prograde, an orbit in the same direction as the planet rotates. Inclinations greater than 90° describe retrograde orbits, thus, An inclination of 0° means the orbiting body has a prograde orbit in the planets equatorial plane. An inclination greater than 0° and less than 90° also describe prograde orbits, an inclination of 63. 4° is often called a critical inclination, when describing artificial satellites orbiting the Earth, because they have zero apogee drift. An inclination of exactly 90° is an orbit, in which the spacecraft passes over the north and south poles of the planet. An inclination greater than 90° and less than 180° is a retrograde orbit, an inclination of exactly 180° is a retrograde equatorial orbit. For gas giants, the orbits of moons tend to be aligned with the giant planets equator, the inclination of exoplanets or members of multiple stars is the angle of the plane of the orbit relative to the plane perpendicular to the line-of-sight from Earth to the object. An inclination of 0° is an orbit, meaning the plane of its orbit is parallel to the sky. An inclination of 90° is an orbit, meaning the plane of its orbit is perpendicular to the sky

10.
Longitude of the ascending node
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The longitude of the ascending node is one of the orbital elements used to specify the orbit of an object in space. It is the angle from a direction, called the origin of longitude, to the direction of the ascending node. The ascending node is the point where the orbit of the passes through the plane of reference. Commonly used reference planes and origins of longitude include, For a geocentric orbit, Earths equatorial plane as the plane. In this case, the longitude is called the right ascension of the ascending node. The angle is measured eastwards from the First Point of Aries to the node, for a heliocentric orbit, the ecliptic as the reference plane, and the First Point of Aries as the origin of longitude. The angle is measured counterclockwise from the First Point of Aries to the node, the angle is measured eastwards from north to the node. pp.40,72,137, chap. In the case of a star known only from visual observations, it is not possible to tell which node is ascending. In this case the orbital parameter which is recorded is the longitude of the node, Ω, here, n=<nx, ny, nz> is a vector pointing towards the ascending node. The reference plane is assumed to be the xy-plane, and the origin of longitude is taken to be the positive x-axis, K is the unit vector, which is the normal vector to the xy reference plane. For non-inclined orbits, Ω is undefined, for computation it is then, by convention, set equal to zero, that is, the ascending node is placed in the reference direction, which is equivalent to letting n point towards the positive x-axis. Kepler orbits Equinox Orbital node perturbation of the plane can cause revolution of the ascending node

11.
Argument of periapsis
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The argument of periapsis, symbolized as ω, is one of the orbital elements of an orbiting body. Parametrically, ω is the angle from the ascending node to its periapsis. For specific types of orbits, words such as perihelion, perigee, periastron, an argument of periapsis of 0° means that the orbiting body will be at its closest approach to the central body at the same moment that it crosses the plane of reference from South to North. An argument of periapsis of 90° means that the body will reach periapsis at its northmost distance from the plane of reference. Adding the argument of periapsis to the longitude of the ascending node gives the longitude of the periapsis, however, especially in discussions of binary stars and exoplanets, the terms longitude of periapsis or longitude of periastron are often used synonymously with argument of periapsis. In the case of equatorial orbits, the argument is strictly undefined, where, ex and ey are the x- and y-components of the eccentricity vector e. In the case of circular orbits it is assumed that the periapsis is placed at the ascending node. Kepler orbit Orbital mechanics Orbital node

12.
Density
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The density, or more precisely, the volumetric mass density, of a substance is its mass per unit volume. The symbol most often used for density is ρ, although the Latin letter D can also be used. Mathematically, density is defined as mass divided by volume, ρ = m V, where ρ is the density, m is the mass, and V is the volume. In some cases, density is defined as its weight per unit volume. For a pure substance the density has the numerical value as its mass concentration. Different materials usually have different densities, and density may be relevant to buoyancy, purity, osmium and iridium are the densest known elements at standard conditions for temperature and pressure but certain chemical compounds may be denser. Thus a relative density less than one means that the floats in water. The density of a material varies with temperature and pressure and this variation is typically small for solids and liquids but much greater for gases. Increasing the pressure on an object decreases the volume of the object, increasing the temperature of a substance decreases its density by increasing its volume. In most materials, heating the bottom of a results in convection of the heat from the bottom to the top. This causes it to rise relative to more dense unheated material, the reciprocal of the density of a substance is occasionally called its specific volume, a term sometimes used in thermodynamics. Density is a property in that increasing the amount of a substance does not increase its density. Archimedes knew that the irregularly shaped wreath could be crushed into a cube whose volume could be calculated easily and compared with the mass, upon this discovery, he leapt from his bath and ran naked through the streets shouting, Eureka. As a result, the term eureka entered common parlance and is used today to indicate a moment of enlightenment, the story first appeared in written form in Vitruvius books of architecture, two centuries after it supposedly took place. Some scholars have doubted the accuracy of this tale, saying among other things that the method would have required precise measurements that would have been difficult to make at the time, from the equation for density, mass density has units of mass divided by volume. As there are units of mass and volume covering many different magnitudes there are a large number of units for mass density in use. The SI unit of kilogram per metre and the cgs unit of gram per cubic centimetre are probably the most commonly used units for density.1,000 kg/m3 equals 1 g/cm3. In industry, other larger or smaller units of mass and or volume are often more practical, see below for a list of some of the most common units of density

13.
Escape velocity
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The escape velocity from Earth is about 11.186 km/s at the surface. More generally, escape velocity is the speed at which the sum of a kinetic energy. With escape velocity in a direction pointing away from the ground of a massive body, once escape velocity is achieved, no further impulse need be applied for it to continue in its escape. When given a speed V greater than the speed v e. In these equations atmospheric friction is not taken into account, escape velocity is only required to send a ballistic object on a trajectory that will allow the object to escape the gravity well of the mass M. The existence of escape velocity is a consequence of conservation of energy, by adding speed to the object it expands the possible places that can be reached until with enough energy they become infinite. For a given gravitational potential energy at a position, the escape velocity is the minimum speed an object without propulsion needs to be able to escape from the gravity. Escape velocity is actually a speed because it does not specify a direction, no matter what the direction of travel is, the simplest way of deriving the formula for escape velocity is to use conservation of energy. Imagine that a spaceship of mass m is at a distance r from the center of mass of the planet and its initial speed is equal to its escape velocity, v e. At its final state, it will be a distance away from the planet. The same result is obtained by a calculation, in which case the variable r represents the radial coordinate or reduced circumference of the Schwarzschild metric. All speeds and velocities measured with respect to the field, additionally, the escape velocity at a point in space is equal to the speed that an object would have if it started at rest from an infinite distance and was pulled by gravity to that point. In common usage, the point is on the surface of a planet or moon. On the surface of the Earth, the velocity is about 11.2 km/s. However, at 9,000 km altitude in space, it is less than 7.1 km/s. The escape velocity is independent of the mass of the escaping object and it does not matter if the mass is 1 kg or 1,000 kg, what differs is the amount of energy required. For an object of mass m the energy required to escape the Earths gravitational field is GMm / r, a related quantity is the specific orbital energy which is essentially the sum of the kinetic and potential energy divided by the mass. An object has reached escape velocity when the orbital energy is greater or equal to zero

14.
Pan-STARRS
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By detecting differences from previous observations of the same areas of the sky, it is expected to discover a very large number of new asteroids, comets, variable stars and other celestial objects. Its primary mission is to detect objects that threaten impact events and is expected to create a database of all objects visible from Hawaii down to apparent magnitude 24. Pan-STARRS is funded in part by the United States Air Force through their Research Labs. Pan-STARRS NEO survey searches all the sky north of declination −47.5, the first Pan-STARRS telescope is located at the summit of Haleakalā on Maui, Hawaii, and went online on December 6,2008, under the administration of the University of Hawaii. Consortium observations for the all sky survey were completed in April 2014, Telescope construction is funded by the U. S. Air Force. Completing the array of four telescopes is estimated at a total cost of US$100 million for the entire array, as of mid-2014, PS2 was in the process of being commissioned. In the wake of substantial funding problems, no clear timeline existed for additional telescopes beyond the second, Pan-STARRS currently consists of two 1.8 m Ritchey–Chrétien telescopes located at Haleakala in Hawaii. The initial telescope, PS1, saw first light using a camera in June 2006. The telescope has a 3° field of view, which is large for telescopes of this size. The focal plane has 60 separately mounted close packed CCDs arranged in an 8 ×8 array, the corner positions are not populated, because the optics do not illuminate the corners. Each CCD device, called an Orthogonal Transfer Array, has 4800 ×4800 pixels, separated into 64 cells and this gigapixel camera or GPC saw first light on August 22,2007, imaging the Andromeda Galaxy. After initial technical difficulties that were mostly solved, PS1 began full operation on May 13,2010. Nick Kaiser, principal investigator of the Pan-STARRS project, summed it up saying “PS1 has been taking science-quality data for six months, the PS1 images however remain slightly less sharp than initially planned, which significantly affects some scientific uses of the data. Each image requires about 2 gigabytes of storage and exposure times will be 30 to 60 seconds, since images will be taken on a continuous basis, it is expected that 10 Terabytes of data will be acquired by PS4 every night. Comparing against a database of known unvarying objects compiled from earlier observations will yield objects of interest, anything that has changed brightness and/or position for any reason. At this time, all funds have been committed The very large field of view of the telescope and the short exposure times enable approximately 6000 square degrees of sky to be imaged every night. Given the need to avoid times when the Moon is bright, this means that an equivalent to the entire sky will be surveyed four times a month. By the end of its initial mission in April 2014

15.
Max Planck Institute for Solar System Research
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The exploration of the solar system is the central theme for research done at this institute. MPS is a part of the Max Planck Society, which operates 80 research facilities in Germany, over the last five years, members of the Institute have each year published about 270 articles in international journals and books and given 360 conference presentations. MPS is organised in three departments, Sun and Heliosphere Planets and Comets Solar and Stellar Interiors In addition, since 2002 there is also an International Max Planck Research School, subjects of research at the Institute are the various objects within the solar system. A major area of concerns the Sun, its atmosphere. The second area of research involves the interiors, surfaces, atmospheres, ionospheres, a further essential part of the activities at the Institute is the development and construction of instruments for space missions. The analysis and interpretation of the datasets are accompanied by intensive theoretical work. Physical models are proposed and then tested and further developed with the aid of computer simulations, the researchers at the MPS are studying the complete range of dynamic and often spectacular processes occurring on the Sun – from the interior to the outer heliosphere. At the heart of research is the magnetic field, which plays a decisive role in these processes. It is generated by gas currents in the interior of the Sun and causes, among other things, answers to the following questions are being sought, Why does the magnetic field change with an eleven-year cycle. How does the field produce the various structures on the Sun. How is the corona heated to millions of degrees. Another important research topic at the The Sun and Heliosphere department is the influence on the Earth due to the Sun’s variable activity. The physical processes involved in the origin and development of fields on the Sun take place on very small scales. The balloon-borne telescope Sunrise, built under Institute leadership and flown in June 2009, was able to make out structures on the Sun’s surface as small as 100 kilometers, future projects will stress research into the physical causes of the Sun’s variations. The Institute develops scientific instruments that fly with spacecraft to other planets, highly specialized cameras have investigated the Saturn moon Titan, analyse the surface of Mars, and probe the clouds and winds of Venus. Microwave instruments determine the composition of atmospheres while infrared spectrometers examine surface rocks, a novel laser altimeter on board BepiColumbo will survey the topography of Mercury to within a meter. Further MPS instruments identify the atoms, electrons, and dust that move around the planets, here the influence of the solar wind on the atmospheric gases is of particular interest. Theoretical studies and intensive computer simulations help to understand the processes both inside and surrounding the planets and to interpret the measured data, in addition, the Institute has along tradition in cometary research

16.
Sloan Digital Sky Survey
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The project was named after the Alfred P. Sloan Foundation, which contributed significant funding. Data collection began in 2000, and the imaging data release covers over 35% of the sky, with photometric observations of around 500 million objects. Data release 8, released in January 2011, includes all photometric observations taken with the SDSS imaging camera, covering 14,555 square degrees on the sky. Data release 9, released to the public on 31 July 2012, includes the first results from the Baryon Oscillation Spectroscopic Survey spectrograph, over 500,000 of the new spectra are of objects in the Universe 7 billion years ago. DR10 also includes over 670,000 new BOSS spectra of galaxies, the publicly available images from the survey were made between 1998 and 2009. SDSS uses a dedicated 2. 5-m wide-angle optical telescope, from 2000-2009 it observed in both imaging and spectroscopic modes, the imaging camera was retired in late 2009, since when the telescope has observed entirely in spectroscopic mode. Images were taken using a system of five filters. For imaging observations, the SDSS telescope used the drift scanning technique and this method allows consistent astrometry over the widest possible field, and minimises overheads from reading out the detectors. The disadvantage is minor distortion effects, the telescopes imaging camera is made up of thirty CCD chips each with a resolution of 2048×2048 pixels, totaling approximately 120 Megapixels. The chips are arranged in five rows of six chips, the filters are placed on the camera in the order r, i, u, z, g. To reduce noise the camera is cooled to 190 kelvin by liquid nitrogen, using these photometric data, stars, galaxies, and quasars are also selected for spectroscopy. The spectrograph operates by feeding an optical fibre for each target through a hole drilled in an aluminum plate. Each hole is positioned specifically for a target, so every field in which spectra are to be acquired requires a unique plate. The original spectrograph attached to the telescope was capable of recording 640 spectra simultaneously, over the course of each night, between six and nine plates are typically used for recording spectra. In spectroscopic mode, the tracks the sky in the standard way. Every night the telescope produces about 200 GB of data and it also obtained repeated imaging of a 300 square degree stripe in the southern Galactic cap. The survey covers over 7,500 square degrees of the Northern Galactic Cap with data from nearly 2 million objects and spectra from over 800,000 galaxies and 100,000 quasars. The information on the position and distance of the objects has allowed the structure of the Universe, with its voids and filaments

17.
P/2010 A2
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P/2010 A2 is a small Solar System body that displayed characteristics of both an asteroid and a comet, and thus, was initially given a cometary designation. Because it has the orbit of an asteroid and showed the tail of a comet. This was the first time a collision had been observed, since then, minor planet 596 Scheila has also been seen to undergo a collision. The position of the nucleus was remarkable for being offset from the axis of the tail and outside of the dust halo, the tail is created by millimeter-sized particles being pushed back by solar radiation pressure. P/2010 A2 was discovered on January 6,2010 by Lincoln Near-Earth Asteroid Research using a 1-meter reflecting telescope with a CCD camera and it was LINEARs 193rd comet discovery. It has been observed over a 112-day arc of the 3.5 year orbit and it appears to have come to perihelion around the start of December 2009, about a month before it was discovered. With an aphelion of only 2.6 AU, P/2010 A2 spends all of its time inside of the frostline at 2.7 AU, beyond the frostline volatile ices are generally more common. Early observations did not detect water vapor or other gases, within less than a month of its discovery it was doubtful that the tail of P/2010 A2 was generated via active outgassing from sublimation of ices hidden beneath the crust. Early modeling indicated that the asteroid became active in late March 2009, reached maximum activity in early June 2009, P/2010 A2 is likely about 150 meters in diameter. Even when it was discovered it was suspected of being less than 500 meters in diameter, another object, centaur 60558 Echeclus in 2006, was suspected of outgassing as a result of an undetermined splitting event. The orbit of P/2010 A2 is consistent with membership in the Flora asteroid family, the Flora family of asteroids may be the source of the Chicxulub impactor, the likely culprit in the extinction of the dinosaurs

18.
The Astronomical Journal
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The Astronomical Journal is a peer-reviewed monthly scientific journal owned by the American Astronomical Society and currently published by IOP Publishing. It is one of the journals for astronomy in the world. Until 2008, the journal was published by the University of Chicago Press on behalf of the American Astronomical Society, the other two publications of the society, the Astrophysical Journal and its supplement series, followed in January 2009. The journal was established in 1849 by Benjamin A. Gould and it ceased publication in 1861 due to the American Civil War, but resumed in 1885. Between 1909 and 1941 the journal was edited in Albany, New York, in 1941, editor Benjamin Boss arranged to transfer responsibility for the journal to the American Astronomical Society. The first electronic edition of The Astronomical Journal was published in January,1998, with the July,2006 issue, The Astronomical Journal began e-first publication, an electronic version of the journal released independently of the hardcopy issues. Its current editor-in-chief is John Gallagher III, 2005–present John Gallagher III 1984–2004 Paul W. Hodge 1980–1983 N. H. Baker 1975–1979 N. H. Baker and L. B. The Astronomical Almanac Official website Dudley Observatory, The Astronomical Journal Scanned issues from ADS

19.
ArXiv
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In many fields of mathematics and physics, almost all scientific papers are self-archived on the arXiv repository. Begun on August 14,1991, arXiv. org passed the half-million article milestone on October 3,2008, by 2014 the submission rate had grown to more than 8,000 per month. The arXiv was made possible by the low-bandwidth TeX file format, around 1990, Joanne Cohn began emailing physics preprints to colleagues as TeX files, but the number of papers being sent soon filled mailboxes to capacity. Additional modes of access were added, FTP in 1991, Gopher in 1992. The term e-print was quickly adopted to describe the articles and its original domain name was xxx. lanl. gov. Due to LANLs lack of interest in the rapidly expanding technology, in 1999 Ginsparg changed institutions to Cornell University and it is now hosted principally by Cornell, with 8 mirrors around the world. Its existence was one of the factors that led to the current movement in scientific publishing known as open access. Mathematicians and scientists regularly upload their papers to arXiv. org for worldwide access, Ginsparg was awarded a MacArthur Fellowship in 2002 for his establishment of arXiv. The annual budget for arXiv is approximately $826,000 for 2013 to 2017, funded jointly by Cornell University Library, annual donations were envisaged to vary in size between $2,300 to $4,000, based on each institution’s usage. As of 14 January 2014,174 institutions have pledged support for the period 2013–2017 on this basis, in September 2011, Cornell University Library took overall administrative and financial responsibility for arXivs operation and development. Ginsparg was quoted in the Chronicle of Higher Education as saying it was supposed to be a three-hour tour, however, Ginsparg remains on the arXiv Scientific Advisory Board and on the arXiv Physics Advisory Committee. The lists of moderators for many sections of the arXiv are publicly available, additionally, an endorsement system was introduced in 2004 as part of an effort to ensure content that is relevant and of interest to current research in the specified disciplines. Under the system, for categories that use it, an author must be endorsed by an established arXiv author before being allowed to submit papers to those categories. Endorsers are not asked to review the paper for errors, new authors from recognized academic institutions generally receive automatic endorsement, which in practice means that they do not need to deal with the endorsement system at all. However, the endorsement system has attracted criticism for allegedly restricting scientific inquiry, perelman appears content to forgo the traditional peer-reviewed journal process, stating, If anybody is interested in my way of solving the problem, its all there – let them go and read about it. The arXiv generally re-classifies these works, e. g. in General mathematics, papers can be submitted in any of several formats, including LaTeX, and PDF printed from a word processor other than TeX or LaTeX. The submission is rejected by the software if generating the final PDF file fails, if any image file is too large. ArXiv now allows one to store and modify an incomplete submission, the time stamp on the article is set when the submission is finalized

20.
ESA
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The European Space Agency is an intergovernmental organisation of 22 member states, dedicated to the exploration of space. Established in 1975 and headquartered in Paris, France, ESA has a staff of about 2,000. The main European launch vehicle Ariane 5 is operated through Arianespace with ESA sharing in the costs of launching, after World War II, many European scientists left Western Europe in order to work with the United States. The meeting was attended by representatives from eight countries, including Harrie Massey. The Western European nations decided to have two different agencies, one concerned with developing a system, ELDO, and the precursor of the European Space Agency. The latter was established on 20 March 1964 by an agreement signed on 14 June 1962, from 1968 to 1972, ESRO launched seven research satellites. ESA in its current form was founded with the ESA Convention in 1975, ESA has 10 founding member states, Belgium, Denmark, France, Germany, Italy, the Netherlands, Spain, Sweden, Switzerland and the United Kingdom. These signed the ESA Convention in 1975 and deposited the instruments of ratification by 1980, during this interval the agency functioned in a de facto fashion. ESA launched its first major scientific mission in 1975, Cos-B, ESA joined NASA in the IUE, the worlds first high-orbit telescope, which was launched in 1978 and operated very successfully for 18 years. A number of successful Earth-orbit projects followed, and in 1986 ESA began Giotto, its first deep-space mission, to study the comets Halley and Grigg–Skjellerup. Hipparcos, a mission, was launched in 1989 and in the 1990s SOHO, Ulysses. Recent scientific missions in co-operation with NASA include the Cassini–Huygens space probe, as the successor of ELDO, ESA has also constructed rockets for scientific and commercial payloads. Ariane 1, launched in 1979, brought mostly commercial payloads into orbit from 1984 onward, the successor launch vehicle of Ariane 5, the Ariane 6 is already in the definition stage and is envisioned to enter service in the 2020s. The beginning of the new millennium saw ESA become, along with agencies like NASA, JAXA, ISRO, CSA and Roscosmos, one of the major participants in scientific space research. Although ESA had relied on co-operation with NASA in previous decades, especially the 1990s, changed circumstances led to decisions to rely more on itself, a 2011 press issue thus stated, Russia is ESAs first partner in its efforts to ensure long-term access to space. ESA maintains its scientific and research projects mainly for astronomy-space missions such as Corot, launched on 27 December 2006 and this is the reason space exploration is an integral part of overall space activities. It has always been so, and it will be more important in the future. ESA describes its work in two overlapping ways, For the general public the various fields of work are described as Activities, Member states participate to varying degrees in the mandatory and optional space programmes

21.
Phil Plait
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Philip Cary Plait, also known as The Bad Astronomer, is an American astronomer, skeptic, writer and popular science blogger. Plait has worked as part of the Hubble Space Telescope team, images and spectra of astronomical objects and he has written two books, Bad Astronomy and Death from the Skies. He has also appeared in science documentaries, including Phil Plaits Bad Universe on the Discovery Channel. From August 2008 through 2009, he served as President of the James Randi Educational Foundation, additionally, he wrote and hosted episodes of Crash Course Astronomy, which aired its last episode in 2016. Plait grew up in the Washington, D. C. area and he has said he became interested in astronomy when his father brought home a telescope when Plait was 5 years old or so. According to Plait, he aimed it at Saturn that night, one look, and that was it. In 1995, he published observations of a ring of material around a supernova. Plaits work with Grady, et al. resulted in the presentation of images of isolated stellar objects from the Hubble Space Telescope. These results have been used in studies into the properties and structure of dim, young, moderate-size stars, called Herbig Ae/Be stars. After his research contributions, Plait shifted focus to concentrate on educational outreach and he went on to perform web-based public outreach for the Fermi Gamma-ray Space Telescope and other NASA-funded missions while at Sonoma State University from 2000 to 2007. In 2001, he coauthored a paper on increasing accessibility of astronomy education resources and his first book, Bad Astronomy, Misconceptions and Misuses Revealed, from Astrology to the Moon Landing Hoax deals with much the same subject matter as his website. His second book, Death from the Skies, describes ways astronomical events could wipe out life on Earth and was released in October 2008, Plaits work has also appeared in the Encyclopædia Britannica Yearbook of Science and the Future and Astronomy magazine. He is also a frequent guest on the SETI Institutes weekly science radio show Big Picture Science, Plait left the JREF as President to focus on a television project, Phil Plaits Bad Universe on the Discovery Channel. The three-part documentary series first aired in the United States on August 29,2010 but was not picked up as a series and he has appeared in numerous science documentaries and programs including How the Universe Works. Plait began publishing explanatory Internet postings on science in 1993, five years later, Plait established Badastronomy. It received an amount of traffic after Plait criticized a Fox Network special accusing NASA of faking the Apollo missions. Astronomer Michelle Thaller has described Badastronomy. com, as well as Plaits book and essays called Bad Astronomy, in 2005, Plait started the Bad Astronomy blog. In July 2008, it moved to a new host, Discover Magazine, while it is primarily an astronomy blog, Plait also posts about skepticism, pseudoscience, antiscience topics, with occasional personal and political posts

22.
Comet
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Community of Metros is a system of international railway benchmarking. CoMET consists of metro systems from around the world. Each metro has a volume of at least 500 million passengers annually, the four main objectives of CoMET are, To build measures to establish metro best practice. To provide comparative information both for the board and the government. To introduce a system of measures for management and these objectives were discussed in detail at the CoMET Annual Meeting 2016, hosted by SMRT Trains of SMRT Corporation. The meeting was held at Singapore in November 2016, in the UITP conference of 1982, London Underground and Hamburger Hochbahn decided to create a benchmarking exercise to compare their two railways with additional data for other 24 metro systems. The project was successful despite the fact that metros were very different in sizes, structures, however, CoMET used the Key Performance Indicator innovatively to solve the problem. In 1994, the Mass Transit Railway of Hong Kong proposed to London Underground, Berlin U-Bahn, New York City Subway, thus, the metros can exchange performance data and investigate best practice amongst similar heavy metros. These five metros are later known as the Group of Five, over time, other large transit systems joined the group. For example, Mexico City Metro, São Paulo Metro and Tokyo Metro joined in 1996, with eight members in total, the group became known as the Community of Metros. Following the success of the CoMET, the Nova group was created in 1998 as another benchmarking association, the Nova is currently consisted of 14 metro systems from around the world. Later, Moscow Metro joined the CoMET in 1999, madrid Metro transferred from Nova to CoMET in 2004. Santiago Metro and Beijing Subway joined in 2008, taipei Metro was the last member to join the CoMET which also joined in 2010

23.
Comet nucleus
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The nucleus is the solid, central part of a comet, popularly termed a dirty snowball or an icy dirtball. A cometary nucleus is composed of rock, dust, and frozen gases, when heated by the Sun, the gases sublimate and produce an atmosphere surrounding the nucleus known as the coma. The force exerted on the coma by the Suns radiation pressure and solar wind cause an enormous tail to form, a typical comet nucleus has an albedo of 0.04. This is blacker than coal, and may be caused by a covering of dust, comets, or their precursors, formed in the outer Solar System, possibly millions of years before planet formation. How and when formed is debated, with distinct implications for Solar System formation, dynamics. Three-dimensional computer simulations indicate the structural features observed on cometary nuclei can be explained by pairwise low velocity accretion of weak cometesimals. The currently favored creation mechanism is that of the nebular hypothesis, astronomers think that comets originate in both the Oort cloud and the scattered disk. Most cometary nuclei are thought to be no more than about 10 miles across, the largest comets that have come inside the orbit of Saturn are C/2002 VQ94, Hale–Bopp, 29P, 109P/Swift–Tuttle, and 28P/Neujmin. The potato-shaped nucleus of Halleys comet contains equal amounts of ice, during a flyby in September 2001, the Deep Space 1 spacecraft observed the nucleus of Comet Borrelly and found it to be about half the size of the nucleus of Halleys Comet. Borrellys nucleus was also potato-shaped and had a black surface. Like Halleys Comet, Comet Borrelly only released gas from small areas where holes in the crust exposed the ice to sunlight, the nucleus of comet Hale–Bopp was estimated to be 60 ±20 km in diameter. Hale-Bopp appeared bright to the eye because its unusually large nucleus gave off a great deal of dust. The nucleus of P/2007 R5 is probably only 100–200 meters in diameter, the largest centaurs are estimated to be 250 km to 300 km in diameter. Three of the largest would include 10199 Chariklo,2060 Chiron, known comets have been estimated to have an average density of 0.6 g/cm3. Below is a list of comets that have had estimated sizes, densities, about 80% of the Halleys Comet nucleus is water ice, and frozen carbon monoxide makes up another 15%. Much of the remainder is carbon dioxide, methane. Scientists think that comets are chemically similar to Halleys Comet. The nucleus of Halleys Comet is also a dark black

24.
Coma (cometary)
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The coma is the nebulous envelope around the nucleus of a comet, formed when the comet passes close to the Sun on its highly elliptical orbit, as the comet warms, parts of it sublimes. This gives a comet a fuzzy appearance when viewed in telescopes and distinguishes it from stars, the word coma comes from the Greek kome, which means hair and is the origin of the word comet itself. The coma is generally made of ice and comet dust, water dominates up to 90% of the volatiles that outflow from the nucleus when the comet is within 3-4 AU of the Sun. The parent molecule is destroyed primarily through photodissociation and to a smaller extent photoionization. The solar wind plays a role in the destruction of water compared to photochemistry. Larger dust particles are left along the orbital path while smaller particles are pushed away from the Sun into the comets tail by light pressure. Comas typically grow in size as comets approach the Sun, and they can be as large as the diameter of Jupiter, about a month after an outburst in October 2007, comet 17P/Holmes briefly had a tenuous dust atmosphere larger than the Sun. The Great Comet of 1811 also had a coma roughly the diameter of the Sun, even though the coma can become quite large, its size can actually decrease about the time it crosses the orbit of Mars around 1.5 AU from the Sun. At this distance the solar wind becomes strong enough to blow the gas and dust away from the coma, Comets were found to emit X-rays in late-March 1996. This surprised researchers, because X-ray emission is associated with very high-temperature bodies. This ripping off leads to the emission of X-rays and far ultraviolet photons, with basic Earth-surface based telescope and some technique, the size of the Coma can be calculated. Called the drift method, one locks the telescope in position and that time multiplied by the cosine of comets declination, times.25 should equal the comas diameter in arcminutes. If the distance to the comet is known, then the apparent size of the coma can be determined. In 2015, it was noted that the ALICE instrument on the ESA Rosetta spacecraft to comet 67/P, detected hydrogen, oxygen, carbon and nitrogen in the Coma, which they also called the Comets atmosphere. Alice is a spectrograph, and it found that electrons created by UV light were colliding and breaking up molecules of water. OAO-2 discovered large halos of hydrogen gas around comets, space probe Giotto detected hydrogen ions at distance of 7.8 million km away from Halley when it did close flyby of the comet in 1986. A hydrogen gas halo was detected to be 15 times the diameter of Sun and this triggered NASA to point the Pioneer Venus mission at the Comet, and it was determined that the Comet emitting 12 tons of water per second. The hydrogen gas emission has not been detected from Earths surface because those wavelengths are blocked by the atmosphere, the process by which water is broken down into hydrogen and oxygen was studied by the ALICE instrument aboard the Rosetta spacecraft

25.
Comet tail
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A comet tail—and coma—are features visible in comets when they are illuminated by the Sun and may become visible from Earth when a comet passes through the inner Solar System. As a comet approaches the inner Solar System, solar radiation causes the materials within the comet to vaporize and stream out of the nucleus. Separate tails are formed of dust and gases, becoming visible through different phenomena, the dust reflects sunlight directly, most comets are too faint to be visible without the aid of a telescope, but a few each decade become bright enough to be visible to the naked eye. In the outer Solar System, comets remain frozen and are difficult or impossible to detect from Earth due to their small size. As a comet approaches the inner Solar System, solar radiation causes the materials within the comet to vaporize and stream out of the nucleus. The streams of dust and gas each form their own distinct tail, the tail of dust is left behind in the comets orbit in such a manner that it often forms a curved tail called the antitail, only when it seems that it is directed towards the Sun. At the same time, the ion tail, made of gases, the ion tail follows the magnetic field lines rather than an orbital trajectory. Parallax viewing from the Earth may sometimes mean the tails appear to point in opposite directions. While the solid nucleus of comets is generally less than 50 km across, the coma may be larger than the Sun, the Ulysses spacecraft made an unexpected pass through the tail of the comet C/2006 P1, on February 3,2007. Evidence of the encounter was published in the October 1,2007 issue of the Astrophysical Journal, the observation of antitails contributed significantly to the discovery of solar wind. The ion tail is the result of ultraviolet radiation ejecting electrons off particles in the coma, once the particles have been ionised, they form a plasma which in turn induces a magnetosphere around the comet. The comet and its magnetic field form an obstacle to outward flowing solar wind particles. The comet is supersonic relative to the wind, so a bow shock is formed upstream of the comet. In this bow shock, large concentrations of cometary ions congregate, the field lines drape around the comet forming the ion tail. If the ion tail loading is sufficient, then the field lines are squeezed together to the point where, at some distance along the ion tail. This leads to a disconnection event. This has been observed on a number of occasions, notable among which was on the 20th, april 2007 when the ion tail of comet Encke was completely severed as the comet passed through a coronal mass ejection. This event was observed by the STEREO spacecraft, a disconnection event was also seen with C/2009 R1 on May 26,2010

26.
Antitail
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An antitail is a spike projecting from a comets coma which seems to go towards the Sun, and thus geometrically opposite to the other tails, the ion tail and the dust tail. However, this phenomenon is an illusion that is seen from the Earth. As Earth passes through the orbital plane, this disc is seen side on. The other side of the disc can sometimes be seen, though it tends to be lost in the dust tail, the antitail is therefore normally visible for a brief interval only when Earth passes through the comets orbital plane. Comet tail The coma and tail at the main Comet article, image of Comet Arend-Roland with prominent antitail Emily Lakdawalla. Spot a comet near Saturn tonight, online Encyclopedia of Science - Antitail

27.
Comet dust
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Comet dust refers to cosmic dust that originates from a comet. Comet dust can provide clues to comets origin, when the Earth passes through a comet dust trail, it can produce a meteor shower. Bulk properties of the comet dust such as density as well as the composition can distinguish between the models. For example, the ratios of comet and of interstellar dust are very similar. The 1) interstellar model says that ices formed on dust grains in the cloud that preceded the Sun. The mix of ice and dust then aggregated into a comet without appreciable chemical modification, J. Mayo Greenberg first proposed this idea in 1986. In the 2) Solar System model, the ices that formed in the interstellar cloud first vaporized as part of the disk of gas. The vaporized ices later resolidified and assembled into comets, so the comets in this model would have a different composition than those comets that were made directly from interstellar ice. The 3) primordial rubble pile model for comet formation says that comets agglomerate in the region where Jupiter was forming, the composition of the dust of comet Wild 2 is similar to the composition of dust found in the outer regions of the accretion disks around newly-forming stars. A comet and its dust allow investigation of the Solar System beyond the main planetary orbits, comets are distinguished by their orbits, long period comets have long elliptical orbits, randomly inclined to the plane of the Solar System, and with periods greater than 200 years. A comet will experience a range of conditions as it traverses its orbit. For long period comets, most of the time it will be so far from the Sun that it will be too cold for evaporation of ices to occur, near the Sun, the heating and evaporation rate will be so great, that no dust can be retained. Therefore, the thickness of dust layers covering the nuclei of a comet can indicate how closely, if a comet has an accumulation of thick dust layers, it may have frequent perihelion passages that dont approach the Sun too closely. The accumulation of dust layers over time would change the character of the short-period comet. A dust layer both inhibits the heating of the cometary ices by the Sun, and slows the loss of gases from the nucleus below

28.
Meteor shower
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A meteor shower is a celestial event in which a number of meteors are observed to radiate, or originate, from one point in the night sky. These meteors are caused by streams of debris called meteoroids entering Earths atmosphere at extremely high speeds on parallel trajectories. Most meteors are smaller than a grain of sand, so almost all of them disintegrate, intense or unusual meteor showers are known as meteor outbursts and meteor storms, which may produce greater than 1000 meteors an hour. The Meteor Data Centre lists about 600 suspected meteor showers of which about 100 are well established, the first great storm in modern times was the Leonids of November 1833. American Denison Olmsted explained the event most accurately, after spending the last weeks of 1833 collecting information he presented his findings in January 1834 to the American Journal of Science and Arts, published in January–April 1834, and January 1836. Work continued, however, coming to understand the nature of showers though the occurrences of storms perplexed researchers. In the 1890s, Irish astronomer George Johnstone Stoney and British astronomer Arthur Matthew Weld Downing, were the first to attempt to calculate the position of the dust at Earths orbit. They studied the dust ejected in 1866 by comet 55P/Tempel-Tuttle in advance of the anticipated Leonid shower return of 1898 and 1899, Meteor storms were anticipated, but the final calculations showed that most of the dust would be far inside of Earths orbit. The same results were independently arrived at by Adolf Berberich of the Königliches Astronomisches Rechen Institut in Berlin, although the absence of meteor storms that season confirmed the calculations, the advance of much better computing tools was needed to arrive at reliable predictions. In 1981 Donald K. Yeomans of the Jet Propulsion Laboratory reviewed the history of showers for the Leonids. A graph from it was adapted and re-published in Sky and Telescope and it showed relative positions of the Earth and Tempel-Tuttle and marks where Earth encountered dense dust. In 1985, E. D. Kondrateva and E. A. Reznikov of Kazan State University first correctly identified the years when dust was released which was responsible for several past Leonid meteor storms, in 1995, Peter Jenniskens predicted the 1995 Alpha Monocerotids outburst from dust trails. In anticipation of the 1999 Leonid storm, Robert H. McNaught, David Asher, in 2006 Jenniskens has published predictions for future dust trail encounters covering the next 50 years. Jérémie Vaubaillon continues to update predictions based on each year for the Institut de Mécanique Céleste et de Calcul des Éphémérides. Because meteor shower particles are all traveling in parallel paths, and at the same velocity and this radiant point is caused by the effect of perspective, similar to parallel railroad tracks converging at a single vanishing point on the horizon when viewed from the middle of the tracks. Meteor showers are almost always named after the constellation from which the appear to originate. This fixed point slowly moves across the sky during the due to the Earth turning on its axis. The radiant also moves slightly from night to night against the stars due to the Earth moving in its orbit around the sun

29.
List of periodic comets
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Periodic comets are comets having orbital periods of less than 200 years or that have been observed during more than a single perihelion passage. Periodic comet is sometimes used to mean any comet with a periodic orbit. Periodic comets receive a permanent number prefix usually after the perihelion passage. In nearly all cases, comets are named after their discoverer, even so, quite a few comets were lost because their orbits are also affected by non-gravitational effects such as the release of gas and other material that forms the comets coma and tail. Unlike a long-period comet, the perihelion passage of a numbered periodic comet can be predicted with a high degree of accuracy. Periodic comets sometimes bear the same name repeatedly, the IAU system distinguishes between them either through the prefix or by the full designation. In the literature, a numbering system is applied to periodic comets, thus 181P and 192P are known as Comet Shoemaker–Levy 6 and Comet Shoemaker–Levy 1. Non-periodic Shoemaker–Levy comets are interleaved in this sequence, C/1991 B1 between 2 and 3, C/1991 T2 between 5 and 6, C/1993 K1 and C/1994 E2 after Shoemaker–Levy 9. In comet nomenclature, the letter before the / is either C, P, D, X, or A for an object that was identified as a comet. Some lists retain the C prefix for comets of periods larger than about 30 years until their return is confirmed

30.
Lost comet
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The D/ designation is for a periodic comet that no longer exists or is deemed to have disappeared. Some astronomers have specialized in this area, such as Brian G. Marsden, there are a number of reasons why a comet might be missed by astronomers during subsequent apparitions. Firstly, cometary orbits may be perturbed by interaction with the giant planets and this, along with nongravitational forces, can result in changes to the date of perihelion. As some comets periodically undergo outbursts or flares in brightness, it may be possible for a faint comet to be discovered during an outburst. Comets can also run out of volatiles and this may have occurred in the case of 5D/Brorsen, which was considered by Marsden to have probably faded out of existence in the late 19th century. Comets are in some known to have disintegrated during their perihelion passage. The best-known example is Bielas Comet, which was observed to split into two components before disappearing after its 1852 apparition, in modern times 73P/Schwassmann–Wachmann has been observed in the process of breaking up. In the case of lost comets this is especially tricky, for example, the comet 177P/Barnard, discovered by Edward Emerson Barnard on June 24,1889, was rediscovered after 116 years in 2006. On July 19,2006, 177P came within 0.36 AU of the Earth, comets can be gone but not considered lost, even though they may not be expected back for hundreds or even thousands of years. With more powerful telescopes it has become possible to observe comets for longer periods of time after perihelion, for example, Comet Hale–Bopp was observable with the naked eye about 18 months after its approach in 1997. It is expected to remain observable with large telescopes until perhaps 2020, comets that have been lost or which have disappeared have names beginning with a D according to current IAU conventions. Comets are typically observed on a periodic return, when they do not they are sometimes found again, while other times they may break up into fragments. These fragments can sometimes be observed, but the comet is no longer expected to return. Other times a comet will not be considered lost until it does not appear at a predicted time, comets may also collide with another object, such as Comet Shoemaker–Levy 9, which collided with Jupiter in 1994

31.
Jupiter-family comet
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A comet is an icy small Solar System body that, when passing close to the Sun, warms and begins to evolve gasses, a process called outgassing. This produces an atmosphere or coma, and sometimes also a tail. These phenomena are due to the effects of radiation and the solar wind acting upon the nucleus of the comet. Comet nuclei range from a few hundred metres to tens of kilometres across and are composed of collections of ice, dust. The coma may be up to 15 times the Earths diameter, if sufficiently bright, a comet may be seen from the Earth without the aid of a telescope and may subtend an arc of 30° across the sky. Comets have been observed and recorded since ancient times by many cultures, Comets usually have highly eccentric elliptical orbits, and they have a wide range of orbital periods, ranging from several years to potentially several millions of years. Short-period comets originate in the Kuiper belt or its associated scattered disc, long-period comets are thought to originate in the Oort cloud, a spherical cloud of icy bodies extending from outside the Kuiper belt to halfway to the nearest star. Long-period comets are set in motion towards the Sun from the Oort cloud by gravitational perturbations caused by passing stars, hyperbolic comets may pass once through the inner Solar System before being flung to interstellar space. The appearance of a comet is called an apparition, Comets are distinguished from asteroids by the presence of an extended, gravitationally unbound atmosphere surrounding their central nucleus. This atmosphere has parts termed the coma and the tail, however, extinct comets that have passed close to the Sun many times have lost nearly all of their volatile ices and dust and may come to resemble small asteroids. Asteroids are thought to have a different origin from comets, having formed inside the orbit of Jupiter rather than in the outer Solar System, the discovery of main-belt comets and active centaur minor planets has blurred the distinction between asteroids and comets. As of November 2014 there are 5,253 known comets, however, this represents only a tiny fraction of the total potential comet population, as the reservoir of comet-like bodies in the outer Solar System is estimated to be one trillion. Roughly one comet per year is visible to the eye, though many of those are faint. Particularly bright examples are called Great Comets, the word comet derives from the Old English cometa from the Latin comēta or comētēs. That, in turn, is a latinisation of the Greek κομήτης, Κομήτης was derived from κομᾶν, which was itself derived from κόμη and was used to mean the tail of a comet. The astronomical symbol for comets is ☄, consisting of a disc with three hairlike extensions. The solid, core structure of a comet is known as the nucleus, cometary nuclei are composed of an amalgamation of rock, dust, water ice, and frozen gases such as carbon dioxide, carbon monoxide, methane, and ammonia. As such, they are described as dirty snowballs after Fred Whipples model

32.
List of hyperbolic comets
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The following is a list of parabolic and hyperbolic comets in the Solar System. Many of these comets may come from the Oort cloud, or perhaps even have interstellar origin, the Oort Cloud is not gravitationally attracted enough to the Sun to form into a fairly thin disk, like the inner Solar System. Thus comets originating from the Oort Cloud can come from any orientation. Comets orbiting in this way still originate from the Solar System, prior to finding a well-determined orbit for comets, the JPL Small-Body Database and the Minor Planet Center list comet orbits as having an assumed eccentricity of 1.0. In the list below, a number of comets discovered by the SOHO space telescope have eccentricities of exactly 1.0, the SOHO satellite observes the corona of the Sun and the area around it, and as a result often observes sungrazing comets, including the Kreutz sungrazers. Although officially given an assumed eccentricity of 1.0, the Kreutz sungrazers have a period of roughly 750 years. The Kreutz sungrazers have a distance of ~0.0050 AU, an inclination of 144 degrees. Some comets in this list are designated with an X-designation, List of comets by type List of periodic comets List of non-periodic comets List of numbered comets List of Halley-type comets List of Solar System objects by greatest aphelion

33.
Great comet
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A great comet is a comet that becomes exceptionally bright. Great comets are rare, on average, only one will appear in a decade, although comets are officially named after their discoverers, great comets are sometimes also referred to by the year in which they appeared great, using the formulation The Great Comet of. The vast majority of comets are never enough to be seen by the naked eye. However, occasionally a comet may brighten to naked eye visibility, the requirements for this to occur are, a large and active nucleus, a close approach to the Sun, and a close approach to the Earth. A comet fulfilling all three of these criteria will certainly be spectacular, sometimes, a comet failing on one criterion will still be extremely impressive. For example, Comet Hale–Bopp had a large and active nucleus. Equally, Comet Hyakutake was a small comet, but appeared bright because it passed extremely close to the Earth. Cometary nuclei vary in size from a few hundreds of metres across or less to many kilometres across, when they approach the Sun, large amounts of gas and dust are ejected by cometary nuclei, due to solar heating. A crucial factor in how bright a comet becomes is how large, the sudden brightening of comet 17P/Holmes in 2007 showed the importance of the activity of the nucleus in the comets brightness. On October 23–24,2007, the comet suffered a sudden outburst which caused it to brighten by factor of about half a million. It unexpectedly brightened from an apparent magnitude of about 17 to about 2.8 in a period of only 42 hours, all these temporarily made comet 17P the largest object in the Solar System although its nucleus is estimated to be only about 3.4 km in diameter. The brightness of a simple reflective body varies with the square of its distance from the Sun. That is, if a distance from the Sun is halved. However, comets behave differently, due to their ejection of large amounts of gas which then also reflect sunlight. Their brightness varies roughly as the cube of their distance from the Sun, meaning that if a comets distance from the Sun is halved. This means that the brightness of a comet depends significantly on its distance from the Sun. For most comets, the perihelion of their orbit lies outside the Earths orbit, any comet approaching the Sun to within 0.5 AU or less may have a chance of becoming a great comet. For a comet to become spectacular, it needs to pass close to the Earth

34.
Sungrazing comet
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A sungrazing comet is a comet that passes extremely close to the Sun at perihelion – sometimes within a few thousand kilometres of the Suns surface. Although small sungrazers can completely evaporate during such an approach to the Sun. However, the evaporation and tidal forces they experience often lead to their fragmentation. Up until the 1880s, it was thought that all bright comets near the sun were the return of a single sungrazing comet. Very little was known about the population of sungrazing comets until 1979 when coronagraphic observations allowed the detection of sungrazers, as of December 12,2013, there are 1488 known comets that come within ~12 solar radii. This accounts for one third of all comets. Most of these objects vaporize during their approach, but a comet with a nucleus radius larger than 2–3 km is likely to survive the perihelion passage with a final radius of ~1 km. Sungrazer comets were some of the earliest observed comets because they can appear very bright, some are even considered Great Comets. This extreme brightening will allow for naked eye observations from Earth depending on how volatile the gases are. One of the first comets to have its orbit computed was the comet of 1680. It was observed by Isaac Newton and he published the results in 1687. However, this marked the first time that it was hypothesized that Great Comets were related or perhaps the same comet, later Johann Franz Encke computed the orbit of C/1680 V1 and found a period near 9000 years and concluded that Cassinis theory of short period sungrazers was flawed. C/1680 V1 had the smallest measured perihelion distance until 1826 with comet C/1826 U1, advances were made in understanding sungrazing comets in the 19th century with the Great Comets of 1843, C/1880 C1, and 1882. He also hypothesized that the parent body was a comet seen by Aristotle, Comet C/1882 R1 appeared only two years after the previously observed sungrazer so this convinced astronomers that these bright comets were not all the same object. Some astronomers theorized that the comet might pass through a resisting medium near the sun, when astronomers observed C/1882 R1, they measured the period before and after perihelion and saw no shortening in the period which disproved the theory. After perihelion this object was seen to split into several fragments. In an attempt to link the 1843 and 1880 comets to the comet in 1106 and 371 BC, Kreutz measured the fragments of the 1882 comet and he then designated that all sungrazing comets with similar orbital characteristics as these few comets would be part of the Kreutz Group. The 19th century also provided the first spectrum taken of a comet near the sun which was taken by Finlay & Elkin in 1882, later the spectrum was analyzed and Fe and Ni spectral lines were confirmed

35.
Kreutz sungrazer
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The Kreutz sungrazers are a family of sungrazing comets, characterized by orbits taking them extremely close to the Sun at perihelion. They are believed to be fragments of one large comet that broke up several centuries ago and are named for German astronomer Heinrich Kreutz, several members of the Kreutz family have become great comets, occasionally visible near the Sun in the daytime sky. The most recent of these was Comet Ikeya–Seki in 1965, which may have one of the brightest comets in the last millennium. It has been suggested that another cluster of bright Kreutz system comets may begin to arrive in the inner Solar System in the few years to decades. Many hundreds of members of the family, some only a few meters across, have been discovered since the launch of the SOHO satellite in 1995. None of these smaller comets have survived its perihelion passage, larger sungrazers such as the Great Comet of 1843 and C/2011 W3 have survived their perihelion passage. Amateur astronomers have been successful at discovering Kreutz comets in the data available in time via the Internet. The first comet whose orbit had been found to take it close to the Sun was the Great Comet of 1680. This comet was found to have passed just 200,000 km above the Suns surface and it thus became the first known sungrazing comet. Its perihelion distance was just 1.3 solar radii, astronomers at the time, including Edmond Halley, speculated that this comet was a return of a bright comet seen close to the Sun in the sky in 1106. 163 years later, the Great Comet of 1843 appeared and also passed close to the Sun. Despite orbital calculations showing that it had a period of several centuries, a bright comet seen in 1880 was found to be travelling on an almost identical orbit to that of 1843, as was the subsequent Great Comet of 1882. An alternative suggestion was that the comets were all fragments of an earlier Sun-grazing comet and this idea was first proposed in 1880, and its plausibility was amply demonstrated when the Great Comet of 1882 broke up into several fragments after its perihelion passage. In 1888, Heinrich Kreutz published a paper showing that the comets of 1843,1880, the comet of 1680 proved to be unrelated to this family of comets. After another Kreutz sungrazer was seen in 1887, the one did not appear until 1945. Two further sungrazers appeared in the 1960s, Comet Pereyra in 1963 and Comet Ikeya–Seki, which became bright in 1965. The appearance of two Kreutz Sungrazers in quick succession inspired further study of the dynamics of the group, the group generally has an Inclination of roughly 140 degrees, a perihelion distance of around 0.01 AU, and a Longitude of ascending node of 340–10°. The brightest members of the Kreutz sungrazers have been spectacular, easily visible in the daytime sky, the three most impressive have been the Great Comet of 1843, the Great Comet of 1882 and Comet Ikeya–Seki

Hubble Space Telescope
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The Hubble Space Telescope is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. Although not the first space telescope, Hubble is one of the largest and most versatile, with a 2. 4-meter mirror, Hubbles four main instruments observe in the near ultraviolet, visible, and near infrared spectra. Hubbles orbit o

1.
The Hubble Space Telescope as seen from the departing Space Shuttle Atlantis, flying Servicing Mission 4 (STS-125), the fifth and final human spaceflight to it.

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Grinding of Hubble's primary mirror at Perkin-Elmer, March 1979

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The backup mirror, by Kodak; its inner support structure can be seen because it is not coated with a reflective surface.

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The OTA, metering truss, and secondary baffle are visible in this image of Hubble during early construction.

Minor planet
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A minor planet is an astronomical object in direct orbit around the Sun that is neither a planet nor exclusively classified as a comet. Minor planets can be dwarf planets, asteroids, trojans, centaurs, Kuiper belt objects, as of 2016, the orbits of 709,706 minor planets were archived at the Minor Planet Center,469,275 of which had received permanen

1.
Only very few minor planets are named. The vast majority is either numbered or still has a provisional designation (blue).

Asteroid
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Asteroids are minor planets, especially those of the inner Solar System. The larger ones have also been called planetoids and these terms have historically been applied to any astronomical object orbiting the Sun that did not show the disc of a planet and was not observed to have the characteristics of an active comet. As minor planets in the outer

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253 Mathilde, a C-type asteroid measuring about 50 kilometres (30 mi) across, covered in craters half that size. Photograph taken in 1997 by the NEAR Shoemaker probe.

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2013 EC, shown here in radar images, has a provisional designation

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⚵

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243 Ida and its moon Dactyl. Dactyl is the first satellite of an asteroid to be discovered.

Main-belt comet
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Main-belt comets are bodies orbiting within the asteroid belt that have shown comet-like activity during part of their orbit. The Jet Propulsion Laboratory defines a main-belt asteroid as an asteroid with an axis of more than 2 AU but less than 3.2 AU. The first main-belt comet discovered is 7968 Elst–Pizarro and it was discovered in 1979 and was f

Perihelion and aphelion
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The perihelion is the point in the orbit of a celestial body where it is nearest to its orbital focus, generally a star. It is the opposite of aphelion, which is the point in the orbit where the body is farthest from its focus. The word perihelion stems from the Ancient Greek words peri, meaning around or surrounding, aphelion derives from the prep

1.
The perihelion and aphelion are the nearest and farthest points (apsides) of a body's direct orbit around the Sun.

Astronomical unit
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The astronomical unit is a unit of length, roughly the distance from Earth to the Sun. However, that varies as Earth orbits the Sun, from a maximum to a minimum. Originally conceived as the average of Earths aphelion and perihelion, it is now defined as exactly 149597870700 metres, the astronomical unit is used primarily as a convenient yardstick f

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Transits of Venus across the face of the Sun were, for a long time, the best method of measuring the astronomical unit, despite the difficulties (here, the so-called " black drop effect ") and the rarity of observations.

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The red line indicates the Earth-Sun distance, which is on average about 1 astronomical unit.

Orbital eccentricity
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The orbital eccentricity of an astronomical object is a parameter that determines the amount by which its orbit around another body deviates from a perfect circle. A value of 0 is an orbit, values between 0 and 1 form an elliptical orbit,1 is a parabolic escape orbit. The term derives its name from the parameters of conic sections and it is normall

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Gravity Simulator plot of the changing orbital eccentricity of Mercury, Venus, Earth, and Mars over the next 50,000 years. The arrows indicate the different scales used. The 0 point on this plot is the year 2007.

Mean anomaly
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In celestial mechanics, the mean anomaly is an angle used in calculating the position of a body in an elliptical orbit in the classical two-body problem. Define T as the time required for a body to complete one orbit. In time T, the radius vector sweeps out 2π radians or 360°. The average rate of sweep, n, is then n =2 π T or n =360 ∘ T, define τ a

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Area swept out per unit time by an object in an elliptical orbit (grey) and by an imaginary object in a circular orbit (red) which completes its orbit in the same period of time. Both sweep out equal areas in equal times, but the angular rate of sweep varies for the elliptical orbit and is constant for the circular orbit.

Orbital inclination
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Orbital inclination measures the tilt of an objects orbit around a celestial body. It is expressed as the angle between a plane and the orbital plane or axis of direction of the orbiting object. For a satellite orbiting the Earth directly above the equator, the plane of the orbit is the same as the Earths equatorial plane. The general case is that

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Fig. 1: One view of inclination i (green) and other orbital parameters

Longitude of the ascending node
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The longitude of the ascending node is one of the orbital elements used to specify the orbit of an object in space. It is the angle from a direction, called the origin of longitude, to the direction of the ascending node. The ascending node is the point where the orbit of the passes through the plane of reference. Commonly used reference planes and

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The longitude of the ascending node.

Argument of periapsis
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The argument of periapsis, symbolized as ω, is one of the orbital elements of an orbiting body. Parametrically, ω is the angle from the ascending node to its periapsis. For specific types of orbits, words such as perihelion, perigee, periastron, an argument of periapsis of 0° means that the orbiting body will be at its closest approach to the centr

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Fig. 1: Diagram of orbital elements, including the argument of periapsis (ω).

Density
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The density, or more precisely, the volumetric mass density, of a substance is its mass per unit volume. The symbol most often used for density is ρ, although the Latin letter D can also be used. Mathematically, density is defined as mass divided by volume, ρ = m V, where ρ is the density, m is the mass, and V is the volume. In some cases, density

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Air density vs. temperature

Escape velocity
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The escape velocity from Earth is about 11.186 km/s at the surface. More generally, escape velocity is the speed at which the sum of a kinetic energy. With escape velocity in a direction pointing away from the ground of a massive body, once escape velocity is achieved, no further impulse need be applied for it to continue in its escape. When given

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Luna 1, launched in 1959, was the first man-made object to attain escape velocity from Earth (see below table).

2.
General

Pan-STARRS
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By detecting differences from previous observations of the same areas of the sky, it is expected to discover a very large number of new asteroids, comets, variable stars and other celestial objects. Its primary mission is to detect objects that threaten impact events and is expected to create a database of all objects visible from Hawaii down to ap

Max Planck Institute for Solar System Research
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The exploration of the solar system is the central theme for research done at this institute. MPS is a part of the Max Planck Society, which operates 80 research facilities in Germany, over the last five years, members of the Institute have each year published about 270 articles in international journals and books and given 360 conference presentat

1.
New Institute building in Göttingen, built in 2013 and occupied in 2014.

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Max Planck Institute for Solar System Research - April 2006

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Max Planck Institute for Solar System Research in Lindau, about a year before relocating - March 2013.

Sloan Digital Sky Survey
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The project was named after the Alfred P. Sloan Foundation, which contributed significant funding. Data collection began in 2000, and the imaging data release covers over 35% of the sky, with photometric observations of around 500 million objects. Data release 8, released in January 2011, includes all photometric observations taken with the SDSS im

P/2010 A2
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P/2010 A2 is a small Solar System body that displayed characteristics of both an asteroid and a comet, and thus, was initially given a cometary designation. Because it has the orbit of an asteroid and showed the tail of a comet. This was the first time a collision had been observed, since then, minor planet 596 Scheila has also been seen to undergo

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Main-belt comet P/2010 A2 as seen in an 8 min photo with a 24" telescope

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Debris field? P/2010 A2 is likely the debris left over from a recent collision between two very small asteroids.

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Comet nucleus Assumed comet nucleus seen to the lower left of debris field

4.
Features

The Astronomical Journal
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The Astronomical Journal is a peer-reviewed monthly scientific journal owned by the American Astronomical Society and currently published by IOP Publishing. It is one of the journals for astronomy in the world. Until 2008, the journal was published by the University of Chicago Press on behalf of the American Astronomical Society, the other two publ

1.
The Astronomical Journal

ArXiv
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In many fields of mathematics and physics, almost all scientific papers are self-archived on the arXiv repository. Begun on August 14,1991, arXiv. org passed the half-million article milestone on October 3,2008, by 2014 the submission rate had grown to more than 8,000 per month. The arXiv was made possible by the low-bandwidth TeX file format, arou

1.
arXiv

2.
A screenshot of the arXiv taken in 1994, using the browser NCSA Mosaic. At the time, HTML forms were a new technology.

ESA
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The European Space Agency is an intergovernmental organisation of 22 member states, dedicated to the exploration of space. Established in 1975 and headquartered in Paris, France, ESA has a staff of about 2,000. The main European launch vehicle Ariane 5 is operated through Arianespace with ESA sharing in the costs of launching, after World War II, m

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ESA Mission Control at ESOC in Darmstadt, Germany

2.
Acronym

3.
ESTEC buildings in Noordwijk, Netherlands. ESTEC was the main technical centre of ESRO and remains so for the successor organization, ESA.

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Mock-up of the Ariane 1

Phil Plait
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Philip Cary Plait, also known as The Bad Astronomer, is an American astronomer, skeptic, writer and popular science blogger. Plait has worked as part of the Hubble Space Telescope team, images and spectra of astronomical objects and he has written two books, Bad Astronomy and Death from the Skies. He has also appeared in science documentaries, incl

3.
The final slide to Plait's presentation at the JREF 's 6th The Amaz!ng Meeting convention

Comet
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Community of Metros is a system of international railway benchmarking. CoMET consists of metro systems from around the world. Each metro has a volume of at least 500 million passengers annually, the four main objectives of CoMET are, To build measures to establish metro best practice. To provide comparative information both for the board and the go

Comet nucleus
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The nucleus is the solid, central part of a comet, popularly termed a dirty snowball or an icy dirtball. A cometary nucleus is composed of rock, dust, and frozen gases, when heated by the Sun, the gases sublimate and produce an atmosphere surrounding the nucleus known as the coma. The force exerted on the coma by the Suns radiation pressure and sol

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The nucleus of Comet Tempel 1.

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Surface of the nucleus of Comet 67P from 10 km away as seen by Rosetta spacecraft

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Tempel 1 and Hartley 2 compared

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Comet Wild 2

Coma (cometary)
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The coma is the nebulous envelope around the nucleus of a comet, formed when the comet passes close to the Sun on its highly elliptical orbit, as the comet warms, parts of it sublimes. This gives a comet a fuzzy appearance when viewed in telescopes and distinguishes it from stars, the word coma comes from the Greek kome, which means hair and is the

1.
Structure of Comet Holmes in infrared, as seen by an infrared space telescope

Comet tail
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A comet tail—and coma—are features visible in comets when they are illuminated by the Sun and may become visible from Earth when a comet passes through the inner Solar System. As a comet approaches the inner Solar System, solar radiation causes the materials within the comet to vaporize and stream out of the nucleus. Separate tails are formed of du

1.
Diagram of a comet showing the dust trail, the dust tail (or antitail) and the ion gas tail, which is formed by the solar wind flow. NASA

2.
Comet Holmes (17P/Holmes) in 2007 showing blue ion tail on right

3.
Comet Lovejoy from orbit

4.
Features

Antitail
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An antitail is a spike projecting from a comets coma which seems to go towards the Sun, and thus geometrically opposite to the other tails, the ion tail and the dust tail. However, this phenomenon is an illusion that is seen from the Earth. As Earth passes through the orbital plane, this disc is seen side on. The other side of the disc can sometime

1.
Comet Lulin antitail to the left, ion tail to right

2.
Showing how a comet may appear to exhibit a short tail pointing in the opposite direction to its type II or dust tail as viewed from Earth i.e. an antitail

3.
Features

Comet dust
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Comet dust refers to cosmic dust that originates from a comet. Comet dust can provide clues to comets origin, when the Earth passes through a comet dust trail, it can produce a meteor shower. Bulk properties of the comet dust such as density as well as the composition can distinguish between the models. For example, the ratios of comet and of inter

1.
Microscopic view of comet dust particle

2.
Features

Meteor shower
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A meteor shower is a celestial event in which a number of meteors are observed to radiate, or originate, from one point in the night sky. These meteors are caused by streams of debris called meteoroids entering Earths atmosphere at extremely high speeds on parallel trajectories. Most meteors are smaller than a grain of sand, so almost all of them d

1.
Four-hour time lapse exposure of sky

2.
Leonids from space

3.
Diagram from 1872

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Meteor shower on chart

List of periodic comets
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Periodic comets are comets having orbital periods of less than 200 years or that have been observed during more than a single perihelion passage. Periodic comet is sometimes used to mean any comet with a periodic orbit. Periodic comets receive a permanent number prefix usually after the perihelion passage. In nearly all cases, comets are named afte

1.
Features

Lost comet
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The D/ designation is for a periodic comet that no longer exists or is deemed to have disappeared. Some astronomers have specialized in this area, such as Brian G. Marsden, there are a number of reasons why a comet might be missed by astronomers during subsequent apparitions. Firstly, cometary orbits may be perturbed by interaction with the giant p

1.
Biela's Comet was seen in two pieces in 1846, and not observed since 1852

2.
5D/Brorsen, which was lost after its 1879 apparition.

3.
Features

Jupiter-family comet
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A comet is an icy small Solar System body that, when passing close to the Sun, warms and begins to evolve gasses, a process called outgassing. This produces an atmosphere or coma, and sometimes also a tail. These phenomena are due to the effects of radiation and the solar wind acting upon the nucleus of the comet. Comet nuclei range from a few hund

List of hyperbolic comets
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The following is a list of parabolic and hyperbolic comets in the Solar System. Many of these comets may come from the Oort cloud, or perhaps even have interstellar origin, the Oort Cloud is not gravitationally attracted enough to the Sun to form into a fairly thin disk, like the inner Solar System. Thus comets originating from the Oort Cloud can c

Great comet
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A great comet is a comet that becomes exceptionally bright. Great comets are rare, on average, only one will appear in a decade, although comets are officially named after their discoverers, great comets are sometimes also referred to by the year in which they appeared great, using the formulation The Great Comet of. The vast majority of comets are

1.
Comet of 1680

2.
The Great Comet of 1577, depicted in a woodcut, over Prague

3.
Halley's Comet 1986 apparition was quite modest compared to some of the brightest

4.
Features

Sungrazing comet
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A sungrazing comet is a comet that passes extremely close to the Sun at perihelion – sometimes within a few thousand kilometres of the Suns surface. Although small sungrazers can completely evaporate during such an approach to the Sun. However, the evaporation and tidal forces they experience often lead to their fragmentation. Up until the 1880s, i

1.
Comet ISON taken with the Wide Field Camera 3 on April 30, 2013.

3.
Features

Kreutz sungrazer
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The Kreutz sungrazers are a family of sungrazing comets, characterized by orbits taking them extremely close to the Sun at perihelion. They are believed to be fragments of one large comet that broke up several centuries ago and are named for German astronomer Heinrich Kreutz, several members of the Kreutz family have become great comets, occasional

1.
An illustration of the sungrazing Great Comet of 1843, as seen from Tasmania

1.
This graphic shows the distance from the Oort cloud to the rest of the Solar System and two of the nearest stars measured in astronomical units. The scale is logarithmic, with each specified distance ten times further out than the previous one.

4.
Simulation: The collision of comet Tempel 1 and the Deep Impact impactor, simulated by Celestia software using pre-impact information. The Sun and the Earth are on the right side. Note: The Deep Impact flyby spacecraft faces the wrong direction. The solar array should face the Sun and the high-gain antenna should point to the Earth.